linux-kernel - Re: [PATCH 2/3] KVM: Unconditionally get a ref to /dev/kvm module when creating a VM

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Message-ID: <YvwPa5GuR8gjQdpc@google.com>
Date:   Tue, 16 Aug 2022 21:43:07 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     David Matlack <dmatlack@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        syzbot+744e173caec2e1627ee0@...kaller.appspotmail.com,
        Oliver Upton <oliver.upton@...ux.dev>
Subject: Re: [PATCH 2/3] KVM: Unconditionally get a ref to /dev/kvm module
 when creating a VM

On Tue, Aug 16, 2022, David Matlack wrote:
> On Tue, Aug 16, 2022 at 05:39:36AM +0000, Sean Christopherson wrote:
> > Unconditionally get a reference to the /dev/kvm module when creating a VM
> > instead of using try_get_module(), which will fail if the module is in
> > the process of being forcefully unloaded.  The error handling when
> > try_get_module() fails doesn't properly unwind all that has been done,
> > e.g. doesn't call kvm_arch_pre_destroy_vm() and doesn't remove the VM
> > from the global list.  Not removing VMs from the global list tends to be
> > fatal, e.g. leads to use-after-free explosions.
> > 
> > The obvious alternative would be to add proper unwinding, but the
> > justification for using try_get_module(), "rmmod --wait", is completely
> > bogus as support for "rmmod --wait", i.e. delete_module() without
> > O_NONBLOCK, was removed by commit 3f2b9c9cdf38 ("module: remove rmmod
> > --wait option.") nearly a decade ago.
> 
> Ah! include/linux/module.h may also need a cleanup then. The comment
> above __module_get() explicitly mentions "rmmod --wait", which is what
> led me to use try_module_get() for commit 5f6de5cbebee ("KVM: Prevent
> module exit until all VMs are freed").

Ugh, I didn't see that one.  The whole thing is a mess.  try_module_get() also
has a comment (just below the "rmmod --wait" comment) saying that it's the one
true way of doing things, but that's at best misleading for cases like this where
a module is taking a reference of _itself_.

The man pages are also woefully out of date :-/